Succinic acid (HOOCCH2CH2COOH), a dicarboxylic acid consisting of 4 carbons, is an organic acid having high utilities, which is widely used as a precursor of medicine, food, cosmetics, and chemical products of other industries (Zeikus et al., Appl. Microbiol. Biotechnol., 51:545, 1999; Song et al., Enzyme Microbial Technol., 39:352, 2006). Particularly, the demand for succinic acid is expected to be dramatically increased as a main source of biodegradable macromolecules, with the latest sharp increase in petroleum prices and increasing interest in relation to environmental pollution (Willke et al., Appl. Microbiol. Biotechnol., 66:131, 2004).
Succinic acid can be produced by chemical synthesis and fermentation, and only a small amount of succinic acid for special use such as use for medicine, food additives and preservatives is produced by traditional microbial fermentation. On the contrary, most succinic acid for industrial use is currently produced through chemical synthesis methods using n-butane and acetylene derived from petroleum or liquified natural gas (LNG) by large chemical companies in America, Europe, Japan and China. Generally, the above-mentioned chemical synthesis methods have a problem of discharging large amounts of hazardous solid wastes, effluents, and waste gases (e.g., CO, etc.) generated during a process of producing succinic acid. Particularly, fossil resources having high possibility of being exhausted are used as a basic material, and thus there is an urgent need to develop a method for preparing succinic acid to replace the fossil resources with alternative ones such as renewable resources.
To overcome these problems caused by the chemical synthesis process for preparing succinic acid, studies on producing succinic acids by microbial fermentation using various renewable resources have been intensively and widely conducted by many researchers. As a result of such efforts, microorganisms, which can produce relatively large amounts of succinic acid, such as genetically engineered Escherichia coli, ruminal bacteria (Actinobacillus, Bacteroides, Mannheimia, Succinimonas, Succinivibrio, etc.) and Anaerobiospirillum, were identified and developed (Song et al., Enzyme Microbial Technol., 39:352, 2006).
As studies for the development of novel strains producing succinic acid using microorganisms, studies using E. coli which can be easily manipulated and whose metabolic characteristics are relatively well studied have been actively conducted. Manipulation of E. coli for the production of succinic acid includes disruption of dh and pfl involved in producing lactic acid and formic acid, manipulation of ptsG, a glucose transporter gene, amplification of sfcA, a malic enzyme gene, introduction of pyc, a foreign gene derived from Rhizobium etli strain involved in pyruvate carboxylation, and disruption of pta, a phosphotransacetylase gene. In addition, genetically engineered E. coli strains, which can produce succinic acid in aerobic conditions, have been developed by manipulating genes involved in lycolysis and tricarboxylic acid(TCA) cycles and lyoxylate pathway (U.S. Pat. No. 5,770,435; Hong et al., Biotechnol. Bioeng., 74:89, 2001; Venuri et al., J. Ind. Microbiol. Biotechnol., 28:325, 2001; U.S. Pat. No. 6,648,061; Lin et al., Eng., 7:116, 2005; Lin et al., Biotechnol. Bioeng., 90:775, 2005).
According to the studies reported to date, Actinobacillus and Mannheimia strains, which are kinds of rumen bacteria, and an obligate anaerobic bacterium, Anaerobiospirillum strain, are known to produce large amounts of succinic acid in anaerobic conditions as well as to have much higher productivity than genetically engineered E. coli. 
With respect to Actinobacillus, Michigan Biotechnology Institute (MBI)-led team of researchers in America isolated Actinobacillus succinogenes 130Z strain (ATCC No. 55618) and developed a method for producing succinic acid (U.S. Pat. No. 5,504,004), and constructed various microbial variants of A. succinogenes using traditional chemical mutagenesis to use in developing a process for producing and purifying succinic acid (U.S. Pat. Nos. 5,521,075; 5,168,055; 5,143,834).
However, succinic acid production processes using microbial fermentation, developed until now, have low productivit, and the yields of succinic acid per gram of carbon source are very low, thus causing difficulties in commercialization, and especially the fermentation-based succinic acid production processes incur huge costs to separate and purify succinic acid from the fermentation broth because succinic acid is produced together with large amounts of various organic acids and ethanol as byproducts during fermentation.
Particularly, A. succinogenes and Anaerobiospirillum succiniciproducens require large amounts of complex nutrients such as yeast extract during fermentation and thus incur high raw material costs for producing succinic acid, and also large amounts of MgCO3 or CaCO3 is added to adjust pH, which causes difficulties in separation and purification (U.S. Pat. Nos. 5,504,004 5,521,075 5,168,055; 5,143,834). To overcome the afore-mentioned shortcomings and commercialize succinic acid production using microbial fermentation, there is an urgent demand to develop a novel succinic acid-producing strain which can effectively produce homo-succinic acid at high yields but prevent byproduct formation to produce homo-succinic acid at high yields (Song et al., Enzyme Microbial Technol., 39:352, 2006).
For the development of a novel succinic acid-producing strain to satisfy the above demands, the isolation of a strain having excellent succinic acid production ability, an understanding of its metabolic characteristic thereof, the completion of its genome sequence thereof, and the establishment of genetic manipulation techniques required for the construction of a genetically engineered strain should be preceded. Although an attempt to try to produce succinic acids by amplifying a phosphoenolpyruvate carboxykinase gene (pckA) of A. succinogenes and A. succiniciproducens in E. coli has been reported (Kim et al., Appl. Environ. Microbiol., 70:1238, 2004; Laivenieks et al., Appl. Environ. Microbiol., 63:2273, 1997), there has been no report on full genome sequence of succinic acid over-producing bacteria, until now, except for Mannheimia succiniciproduces, and there has been no attempt to try to develop a genetically engineered succinic acid producing strain based on genome sequence, except the studies reported by the present inventors.
The present inventors have reported that they isolated M. succiniciproducens MBEL55E (deposited under KCTC accession no. KCTC0769BP) producing succinic acid with high efficiency from the rumen of Korean cow, completed its full genome sequence, and characterized the metabolic properties of the strain (Hong et al., Nature Biotechnol., 22:1275, 2004). Also, the present inventors have constructed a bacterial mutant M. succiniciproducens LPK (deposited under KCTC accession no. KCTC10558BP) by disrupting a gene encoding lactate dehydrogenase (ldhA) and a gene encoding pyruvate formate-lyase (pfl) in M. succiniciproducens MBEL55E. In addition, the present inventors have constructed a microbial variant (M. succinciproducens LPK7 (deposited under KCTC accession no. KCTC1062BP)) by disrupting a phosphotransacetylase gene (pta) and an acetate kinase gene (ackA) in said microbial variant, M. succiniciproducens LPK, thus increasing the production of succinic acid (WO 2005/052135 A1). However, in case of such microbial variants, although the formation of byproducts, lactic acid, formic acid and acetic acid could be suppressed effectively, large amounts of pyruvic acid accumulated as a byproduct during fermentation. Most of all, the growth rate of M. succiniciproducens LPK7 has become so low compared with the wild-type strain that an excellent succinic acid productivity could not be achieved. Although the present inventors have constructed a microbial variant (M. succinciproducens PALK, deposited under KCTC accession no. KCTC10973BP) by disrupting a lactate dehydrogenase-encoding gene (ldhA), a phosphotranacetylase-encoding gene (pta), and an acetate kinase-encoding gene (ackA) in M. succinciproducens MBEL55E (PCT/KR2007/003574), and as a result, achieved an increase in cell growth rate and improvement of succinic acid productivity, there is still a need for development of a better succinic acid producing microbial variant.
The present inventors have made extensive efforts to construct a microbial variant (M. succiniciproducens ALK), by disrupting a lactate dehydrogenase gene (ldhA) and an acetate kinase gene (ackA) in M. succiniciproducens MBEL55E. Variant ALK has been deposited in compliance with the Budapest Treaty on Nov. 29, 2012 at the Korean Collection for Type Cultures located at Korea Research Institute of Bioscience and Biotechnology, 125 Gwahak-ro, Yuseong-gu, Daejeon 305-806, Republic of Korea under deposit accession no. KCTC 12326BP. The deposit will be maintained for at least thirty (30) years and will be irrevocably and without restriction released to the public upon the grant of a patent. The availability of a deposit does not constitute a license to practice the subject invention in derogation of patent rights granted by government action. The present inventors have also constructed a variant (M. succinciproducens ALKt) as a variant having the same effect as that of said ALK variant based on the result of a virtual cell model, and found that when the microbial variants were cultured using glucose as a carbon source in anaerobic conditions, they can produce homo-succinic acid at high yields, thereby completing the present invention.